Every year, it’s the same story. We bring in a few plants in autumn – geraniums, fuchsias or other “annuals” we’d like to keep – and wash them well to keep out unwanted insects. At first, all goes well. Then February arrives, and suddenly dozens, then hundreds of “flying dandruff” appear. This is the whitefly, a group of insects of Aleyrodidae family, and it can devastate your plants. Where does it come from? What does it do in winter? How do you control it? We’ll explain in the following lines.
No Spontaneous Generation!
Although whiteflies often seem to appear out of nowhere in February or March, their sudden appearance is not due to spontaneous generation, as scientists believed in the mid-19th century. In fact, the insect was already present among your plants, but in “diapause”. This state is equivalent to hibernation, and begins in late autumn and continues until early winter for many insects. Then, in spring – and the whitefly’s spring starts earlier than ours, as soon as the days begin to lengthen in February – it wakes up and starts multiplying wildly. Soon your plants are covered with them.
White Flies
Up close, these pieces of “dandruff” turn out to be a tiny white fly with “V”-shaped wings. Its characteristic feature is that when disturbed, it flies off in massive flocks, landing fairly quickly on another plant. The adult lays eggs on the underside of leaves. These tiny, essentially invisible eggs give rise to semi-transparent nymphs that quickly settle under a leaf and then become immobile, not even moving if touched. Shaped like small, almost transparent scales, they stick to the back of leaves, usually in the presence of winged adults.
Whiteflies damage plants in three ways: first and foremost, nymphs (and, to a lesser degree, adults) suck sap from leaves, leaving them weakened and often mottled yellow. If there are many of them, the whole plant withers. Whiteflies can also transmit diseases, such as the mosaic virus, to infested plants. Finally, they emit a transparent, sweet liquid called honeydew, which can run down the lower leaves and onto the floor. Eventually, the honeydew turns black under the effect of a fungus called sooty mold. Sooty mold is not directly harmful to plants, but because of its black coloring, it reduces photosynthesis by the plant, which then runs out of solar energy. In addition, the plant loses all physical appeal. In extreme cases, it weakens and may even die.
Controlling the Plague
Whiteflies are extremely prolific and fairly generalist: almost any plant can be affected, including houseplants, annuals overwintering in the house, seedlings, etc., but they have a clear preference for fuchsias and geraniums. Also, if you dare to grow indoor plants that prefer to be outdoors, such as herbs and tomato plants, they are often affected.
The problem will more or less take care of itself if you take your plants outdoors during the summer, when the whitefly’s natural predators, in addition to wind and rain, reduce the population to an acceptable level. On the other hand, between their awakening in February/March and the plants coming out in June, they have time to make a fine mess.
You can also spray with a product containing insecticidal soap to control them. Apply it to both sides of the leaves of affected plants. As this technique reduces infestation without eliminating it completely, treatments should be repeated, at least on affected plants, on a weekly basis.
Turn Their Weakness to Your Advantage
Whiteflies have a little flaw which can be very useful in their repression: they are attracted by the color yellow. Place yellow sticky traps (available from garden centers) near affected plants and you’ll collect hundreds of them. Unfortunately, there are often enough left to continue causing damage. An even more effective method: paint the end of a hand-held vacuum cleaner yellow and pass it among the plants: the whiteflies will throw themselves at the vacuum cleaner as if it were chocolate… and get sucked in. Obviously, this treatment only affects the adults; the immobile nymphs remain stuck to the underside of the leaves. It must therefore be repeated every 4 to 6 days, as the nymphs moult into adults. After about 3 treatments, the last adults will have been sucked out and the problem will be solved.
A Light Trap
Another, more costly, but less labor-intensive option is to attract whiteflies with a light trap. Such traps consist of a lamp, one or more fluorescent tubes specially designed to attract insects, and a yellow sticky board. Flying insects, including whiteflies, are irresistibly attracted by the light and stick to the yellow cardboard. All that’s left to do is replace the yellow sticky board when it’s covered in insects. With such a device, whitefly problems can be reduced to such an extent that, if they are ever present, it’s in such minute quantities that there’s no need to react.
Act Soon
Whitefly season has unfortunately begun. If you want to have beautiful plants, keep an eye on them now and be prepared to react without delay!
Larry Hodgson published thousands of articles and 65 books over the course of his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings accessible to the public. This text was originally published in Le Soleil on February 19, 2006.
Ugh! Another bug.